Thank you for the help! It makes a big difference.
It's clear I can't have the babies around the muscovies. Will the mallard dad, mom and babies make a happy family, or should I keep the mallard drake out as well?
I wouldn't buy mallards in the future. Rescue/adopt yes. I'm loving the muscovies right now. Having baby mallards born here, for the first time, is exciting though. I can't wait to see how the new family gets along and what it's like when mom and babies get to be together. It's worth the all trouble.
Personally, I would keep the Mallard drake out as well, but you CAN try him out first and just watch him very very carefully for awhile to see what he does. Remove him immediately if the starts to go after or try to grab any of the ducklings. Drakes will often attack and kill ducklings, so it is a danger to have him in with them. That said, some drakes do just fine with ducklings, so it's up to you if you want to give him a chance or not.